The present invention relates to a pressurized water nuclear power plant and a passive cooling and depressurization system therefor.
Most light water reactors (LWRs) have a safety system such as an emergency core cooling system (ECCS). Reactors having an active component such as a pump are called “active safety reactors”. On the other hand, reactors with a safety system that has a passive component such as a tank are called “passive safety reactors”. Known as a passive safety reactor representing pressurized water reactors (PWRs) is AP1000 (see, for example, IAEA-TECDOC-1391, “Status of advanced light water reactor design 2004,” IAEA, May 2004, pp. 279-306; the entire content of which is incorporated herein by reference).
FIG. 7 is a vertical cross sectional view of a containment vessel used in a conventional passive safety PWR (AP1000).
In AP1000, the reactor core 1 is contained in a reactor pressure vessel (RPV) 2. The reactor pressure vessel 2 is connected to two steam generators (SGs) 3 by both a cold leg pipe 4 and a hot leg pipe 5. A reactor coolant pump (RCP) 6 is directly attached to the bottom of the steam generator 3. These devices and pipes, which constitute a reactor pressure boundary, are all contained in a containment vessel (CV) 77.
The containment vessel 77 of AP 1000 is a most typical containment vessel, called “large dry CV”, for use in PWRs. The containment vessel 77 is made of steel, because it is designed to be cooled with the external air in case of an accident. Most PWR plant other than AP1000 rather use a large dry CV made of prestressed concrete.
In the containment vessel, an in-containment refueling water storage tank (IRWST) 8 is provided. The in-containment refueling water storage tank 8 works as a gravity-driven cooling system if a loss-of-coolant accident occurs due to a rupture of the cold leg pipe 4 or the like. This gravity-driven cooling system in cooperation with other passive ECCS floods the lower part of the containment vessel with water to a higher level than the cold leg pipe 4.
After that, it is designed that the recirc screen is opened, introducing the water always into the reactor pressure vessel 2 to cool the fuel in the reactor core safely. If the water introduced into the reactor pressure vessel 2 is heated by the decay heat of the fuel in the reactor core, steam is generated and the steam fills the gas phase of the containment vessel 77 resulting in a rise of the temperature and pressure in the containment vessel 77.
A shield building 71 is built outside the containment vessel 77. A cooling water pool 72 of a passive containment cooling system (PCS) is provided on the top of the shield building 71. The cooling water pool 72 is filled with PCS cooling water 73. In case of a LOCA, the PCS cooling water 73 drains onto the containment vessel 77. Air flows into the shield building 71 through a containment cooling air inlet 74 and then a natural circulation force raises the air through the gap between an air baffle 75 and the wall of the containment vessel 77 until the air is released outside through a containment cooling heated air discharge 76 formed at the top of the shield building 71. The drainage of the PCS cooling water 73 and the natural convection of air serve to cool the containment vessel 77 in safety.
In this way, AP1000 can cool the reactor core 1 and the containment vessel 77 with an extremely high reliability only by the passive safety systems requiring no external power source.
FIG. 8 is a block diagram of a passive residual heat removal system and an automatic depressurization system of the AP1000.
The passive residual heat removal (passive RHR) system of the AP1000 has a passive RHR heat exchanger 61. The passive RHR heat exchanger 61 is submerged in refueling water 66 stored in the in-containment refueling water storage tank 8. The in-containment refueling water storage tank 8 is arranged lower than an operating deck 90. The passive RHR heat exchanger 61 is connected to the hot leg pipe 5 via a coolant supply pipe 62. An inlet valve 63 is equipped on the coolant supply pipe 62. The passive RHR heat exchanger 61 is also connected to the cold leg pipe 4 at a position near an outlet of the steam generator 3 via a coolant return pipe 65. An outlet valve 64 is equipped on the coolant return pipe 65.
The inlet valve 63 is kept open during a normal operation and coolant is constantly supplied to the passive RHR heat exchanger 61 via the coolant supply pipe 62. The outlet valve 64 is kept closed during the normal operation.
The cold leg pipe 4 is connected to the reactor pressure vessel 2 so that the coolant cooled by the steam generator 3 circulates in the reactor pressure vessel 2 with a driving force of the reactor coolant pump 6. Note that the cold leg pipe 4 is shown separately on the right side and the left side in FIG. 8 for convenience. In addition, while a pair of steam generators 3 are provided in a known AP1000 as shown in FIG. 7, only a single steam generator 3 is shown in FIG. 8.
During the normal operation of the power plant, since the outlet valve 64 is closed, coolant in the passive RHR heat exchanger 61 does not circulate in the reactor pressure vessel 2 through the cold leg pipe 4. However, when a transient such as a loss of offsite power or a feed-water pump trip occurs and a supply of secondary cooling water to the steam generator 3 stops, the primary coolant is heated by decay heat that is continuously generated in the reactor core 1. Then, the primary coolant passes through the hot leg pipe 5 due to the driving force of natural circulation and heats the secondary cooling water remaining in the steam generator 3. The primary coolant itself is cooled and increases its density (specific weight) in the steam generator 3 and returns into the reactor pressure vessel 2 via the cold leg pipe 4. The primary coolant that returns to the reactor pressure vessel 2 is then heated again by decay heat of the reactor core 1 and naturally circulates between the reactor pressure vessel 2 and the steam generator 3.
As this process proceeds repeatedly for a while, the secondary cooling water in the steam generator 3 decreases by evaporation and it may become impossible to cool the reactor core 1. Therefore, before it becomes impossible to cool the reactor core 1, the outlet valve 64 of the passive RHR automatically opens with a level low signal of the secondary cooling water in the steam generator 3. Then, as a result, the primary coolant in the passive RHR heat exchanger 61 is driven to circulate in the reactor pressure vessel 2 through the coolant return pipe 65 and the cold leg pipe 4.
Decay heat continuously generated in the reactor core 1 is transferred to the refueling water 66 stored in the in-containment refueling water storage tank 8 via the passive RHR heat exchanger 61 and the refueling water 66 will start evaporating in several hours. Evaporation of refueling water 66 may deteriorate environmental conditions of an atmosphere of the containment vessel 2 and may have adverse effects on normal facilities or equipments such as an electric component. However, the facilities and equipment that are important for safety are designed to withstand such environmental conditions.
The generated steam fills up the containment vessel 77. Then, it is cooled by circulating external air and the passive containment cooling function of the PCCS pool 73. And the condensed water flows into the in-containment refueling water storage tank 8. The PCCS pool 73 contains water sufficient for removing decay heat for three days so that in principle the nuclear reactor can be safely cooled by a combination of the passive RHR and the passive containment cooling function even if a loss of AC power source continues for three days.
However, the driving force of the cooling function is the natural circulation force of the primary coolant due to the decay heat generated in the reactor core 1. Therefore, an amount of removable heat is limited to an amount equivalent to the decay heat and the primary coolant cannot progressively be depressurized and cooled. Thus, if the loss of AC power source continues for a long period, the reactor pressure boundary can be kept at high temperature and high pressure about 150 atm (about 15.5 MPa). Such a condition is called a hot shutdown state of nuclear reactor.
To shut down the nuclear reactor more reliably in safety, it is desirable to depressurize and cool the primary coolant. Such a condition is called a cold shutdown state of nuclear reactor.
In the AP1000, if the loss of AC power source continues for more than twenty-four hours, a timer automatically activates an automatic depressurization system (ADS) in order to achieve the cold shutdown. The automatic depressurization system has four stages. The automatic depressurization system first stage 51, the automatic depressurization system second stage 52 and the automatic depressurization system third stage 53 are arranged on an upper part of pressurizer 80. The automatic depressurization system fourth stage 68 is arranged at a position same as a position where coolant supply pipe 62 branches out from the hot leg pipe 5.
The pressurizer 80 is connected to the hot leg pipe 5 by a riser pipe 81. During the normal operation of the nuclear power plant, the primary coolant is stored in the pressurizer 80 to about a half of its capacity. The primary coolant 82 in the pressurizer 80 is heated to a saturation temperature by a heater during the normal operation. Therefore, saturated steam 83 exists in an upper part of the pressurizer 80.
Automatic activations of the ADS is conducted sequentially with time lags from the automatic depressurization system first stage 51, the automatic depressurization system second sage 52, the automatic depressurization system third stage 53 to the automatic depressurization system fourth stage 68. When the automatic depressurization system first stage 51, the automatic depressurization system second stage 52 and the automatic depressurization system third stage 53 are activated, the saturated steam 83 in the pressurizer 80 flows through a discharge pipe 69, is discharged from the sparger 70 in the in-containment refueling water storage tank 8 and then condensed. If the reactor pressure vessel 2 is depressurized to a certain extent during this process, accumulator 84 automatically starts operation to compensate the discharged primary coolant. The driving force of the accumulator 84 is high-pressure nitrogen gas 86 stored inside. As a result, coolant 85 that is also stored inside is injected into the reactor pressure vessel 2 via an injection valve 87 and an ECCS injection pipe 54. At this time, the nitrogen gas 86 that is driving force also flows into the reactor pressure vessel 2.
As the depressurization of the reactor pressure vessel 2 progresses, the automatic depressurization system fourth stage 68 is finally activated to directly discharge the primary coolant and a complete depressurization of the reactor pressure vessel 2 is taken place. After the internal pressure of the reactor pressure vessel 2 decreases sufficiently, the in-containment refueling water storage tank 8 starts injecting water into the reactor pressure vessel 2 as a gravity-driven ECCS. A cold shutdown state of the nuclear reactor is achieved at this stage of process. Then, as a result, the nuclear reactor is shut down satisfactorily in safety. However, the primary coolant that is continuously flowing out from the automatic depressurization system fourth stage 68 completely submerges a lower part of the containment vessel to the level of the cold leg pipe 4. This makes it difficult to restart the nuclear reactor for the normal power operation shortly after the AC power source becomes available.
The natural circulation force due to decay heat generated by the fuel in the reactor core is employed as the driving force of the passive residual heat removal (RHR) system of conventional passive safety pressurized water reactor such as AP1000. Therefore, it is not possible to remove heat more than the decay heat and the nuclear reactor cannot be brought into a cold shutdown state.
Additionally, in a passive RHR that circulates primary coolant with the natural circulation force from decay heat, it is difficult to put the heat exchanger higher than the highest level of the primary coolant (about same as the normal water level in the pressurizer). More specifically, the heat exchanger is arranged below the operating deck that is located below the normal water level in the pressurizer.
If the heat exchanger is located higher than the water level of the pressurizer, coolant flows not into the heat exchanger but into the pressurizer. Therefore, the position of the heat exchanger needs to be lower than the normal water level of the pressurizer. As a result, the difference of vertical level between the heat exchanger and the reactor core is limited to as small as 10 m and it is difficult to increase an injection head due to gravity.
Additionally, use of the water in the in-containment refueling water storage tank as in the passive RHR of AP1000 causes vaporization of this water and the steam may have adverse effects on normal components in the containment vessel such as the electric equipment. Furthermore, after steam generation in the containment vessel due to boiling of water in the in-containment refueling water storage tank or discharging the primary coolant into the containment vessel because of the activation of the final stage of the automatic depressurization system, it is necessary to cool the containment vessel by the passive containment cooling system (PCS). Thus, a large volume of cooling water needs to be stored in the ceiling section of the shield building.
The volume of cooling water to be stored in the ceiling section of the shield building is not particularly a problem for AP1000 whose electric power output is 1,117 MWe. However, if the electric power output of the plant is about 1,700 MWe, a large volume of cooling water as about 4,500 m3 needs to be stored in the ceiling section of the shield building for continuous cooling of the containment vessel by a passive RHR.